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Celeriac and electricity: dual-use system on arable land

The sun’s energy is harvested on two levels in agrophotovoltaic systems where food can be grown beneath solar collectors. Food or fuel? Potatoes or electricity? In addition to growing energy crops for biofuel and biogas production, open space solar plants also compete with food production when it comes to land use. Agrophotovoltaics (APV), i.e. the dual use of arable land, can mitigate the conflicting interests of agriculture and open space PV systems. Solar panels collect solar energy directly above the crop fields that farmers plough with their tractors. APV-RESOLA is a pilot project aimed at investigating the efficiency of this dual use.

The solar panels are placed at a precisely calculated distance from each other to ensure optimal electricity yield and the best possible exposure of crops to the light that they require. The panels rest on long steel poles at five-metre intervals. While the collectors harvest energy at lofty heights, potatoes and celeriac tubers thrive in the soil directly below. Back in 1981, Prof. Adolf Goetzberger, founder of the Fraunhofer Institute for Solar Energy Systems ISE, published an article in which he proposed this dual-use strategy for the first time. Many people thought it was a ridiculous idea at the time, but the dual use of land has since become reality on a 0.3-hectare field in the Heggelbach Hofgemeinschaft (Heggelbach farm community), a Demeter farm east of the village of Stockach on Lake Constance. The APV plant, funded by the German Federal Ministry of Education and Research, has been up and running since September 2016, and so far everything seems to indicate that agriculture and energy production can be successfully combined on one and the same area.

The solar panels cover an area of 126 x 24 metres (approximately 3000 sqm) and consist of 720 modules with a maximum total output of 194 kilowatts (kWp). The 3000 sqm field below the solar panels is divided into rows where clover grass, potatoes, celeriac and wheat grow. Directly adjacent to this area is another 3000 sqm reference plot where the same plants are cultivated under normal conditions. Organic farmer Florian Reyer of the Heggelbach farm community is responsible for the production of crops. "Sometimes it can get a little tight when we maneuver the tractors between the poles of the solar modules, but in general, growing agricultural produce below the solar panels is not a problem,” Reyer says, examining the celeriac tubers that, as late autumn approaches, will soon be harvested. The farmers are mainly interested in how the solar panels impact the soil and yields. Agricultural scientists Prof. Dr. Petra Högy and Andrea Ehmann from the University of Hohenheim are investigating this by determining harvest quantity and quality, and recording the temperature and humidity of the soil, air, solar radiation and precipitation using more than 30 static measurement stations along with some mobile ones.

"Our results show that the solar panels have hardly any impact on the distribution of precipitation and relatively little sunlight is kept away from the plants," explains Högy. The results of the 2017 harvest have been evaluated and are quite promising. The crop yield of clover grass under the solar panels was only five percent less than that of the reference plot. The yield losses for potatoes, wheat and celeriac were somewhat higher, accounting for between 18 and 19 percent. The researchers believe that in the extremely long, dry summer of 2017, the solar panel shading might even have been beneficial. “We expect that similar harvest yields can be achieved for wheat in the APV field in 2018, and potentially a slightly higher potato yield,” says Andrea Ehmann. This might be due to the fact that the soil beneath the solar panels remained moist for a longer period of time than the field without the panels. However, the evaluations are still ongoing and final results will be available in spring 2019. What can be concluded so far is that positive shading effects might be an additional advantage of APV systems in times of climate change or in hotter, drier regions.

Glossary

A gene is a hereditary unit which has effects on the traits and thus on the phenotype of an organism. Part on the DNA which contains genetic information for the synthesis of a protein or functional RNA (e.g. tRNA).

Being lytic is the feature of a bacteriophage leading to the destruction (lysis) of the host cell upon infection.

Transformation is the natural ability of some species of bacteria to take up free DNA from their surroundings through their cell wall. In genetic engineering, transformation denotes a process which is often used to introduce recombinant plasmids in E. coli, for example. This is a modified version of natural transformation.

In the first twelve months, the APV plant produced over 245,000 kilowatt hours of electricity. This corresponds to 1,260 kilowatt-hours electricity per installed kilowatt, one third more than the Germany-wide average value of 950 kWh/kW. Stephan Schindele, project manager at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, expects a similarly high energy yield for 2018. Taken together, the slightly lower crop yield and the good electricity yield is a fairly positive result. “All in all, the total yield on the APV area could be increased by 60%,” says Schindele, who is quite happy with the initial results. An APV plant might therefore be an interesting additional source of income for farmers.

The electricity produced on the Heggelbach farm community is either directly consumed on the farm or fed into the power grid in cooperation with EWS Schönau, a power producer that is also participating in the project. In cooperation with project partner BayWa r.e., the Heggelbach farm community is testing a 160 kWh battery storage system to find out whether it would be possible to supply the cold storage rooms with self-produced electricity at night. Another idea is to sell the power directly to nearby housing estates.

Consumers prefer APV to biogas and open space PV

The APV plant that consists of 50 tons of steel resting on high poles above the field, is not exactly what you’d call aesthetically pleasing. "At first, some of the locals were extremely skeptical," recalls Florian Reyer. This came as no surprise to him as he also sees the pilot project as dealing with social and bureaucratic hurdles. The Institute for Technology Assessment and Systems Analysis of the Karlsruhe Institute of Technology (KIT) involved the public in workshops before and after construction of the pilot plant. "The workshop participants agreed that all available roof areas should first be completely covered with photovoltaic modules, as this would avoid putting up extra poles and transformer stations,” says Dr. Christine Rösch of the KIT, “if it were still necessary to use arable land for producing energy, the majority of participants would choose electricity from APVs rather than from open space PV systems and biogas plants because it is possible to grow food beneath the APV panels.” However, the erection of APVs would undoubtedly be a massive intrusion on the landscape. The workshop participants fear “APV wild growth” as happened with biogas plants some years ago and “roofing of the landscape”. Another issue of concern was the threat of agriculture becoming “pseudo agriculture”, as generating electricity is simpler, more reliable and more lucrative than harvesting biomass.

Whether APV plants will prevail in the future depends to a great extent on political decisions. Stephan Schindele expects conventional ground-mounted open space PV systems to become profitable over the next few years even without public funding. APV plants might play an important role in reducing the amount of land used for energy production.

Dual use with enormous potential

There is still room for optimising the use of materials. For example, it would be possible to replace the steel scaffolding with more climate-friendly wooden poles. Schindele points out that nobody is currently pursuing this approach as the price of steel is still relatively low. An Austrian engineer, Günter Czaloun, has come up with the idea of attaching the solar panels to a free-floating rope construction, which works well with just four cornerstones1. The basic idea of combining energy and food production still has room for refinement. Stephan Schindele and his colleagues see great potential in nurseries and fruit growers, where solar modules could be used for shading or protection from hail. Greenhouses and film cultures are also suitable for modified APV approaches. In Chile and Vietnam, the Fraunhofer ISE is currently testing smaller plants that make rural areas self-sufficient in local energy.2

For Reyer, photovoltaics is not the only solution: "We need a mix of energy supplies," he says. The Heggelbach farm community is already on the right track. In addition to the APV plant, which will be leased by the farm for 25 years and continue operating once the research project has come to an end, the farm operates a wood gasifier for producing electricity and heat, which is fed with woodchips from the surrounding forests. And of course, the roofs of stables and houses are also equipped with solar panels. Stephan Schindele of the Fraunhofer ISE believes that in the future APV plants in Baden-Württemberg will be no larger than one to two hectares and located in close proximity to a farm or next to industrial estates.

Project details:

The APV-RESOLA project is funded by the German Federal Ministry of Education and Research and the FONA framework for research for sustainable development. It is a joint project of the Fraunhofer Institute for Solar Energy Systems ISE, BayWa r.e. Solar Projects GmbH, Elektrizitätswerke Schönau, Hofgemeinschaft Heggelbach, the Karlsruhe Institute of Technology, the Bodensee-Oberschwaben Regional Association and the University of Hohenheim.